In recent years, laser technology has been remarkably advanced in particular, downsizing and output power up have been made in solid lasers and semiconductor lasers which have light emitting regions in the near infrared to infrared ranges. Accordingly, these lasers are highly useful as exposure light sources in direct plate making from digital data available from computers and so on.
In a negative type lithographic printing plate precursor for infrared lasers with the use of such an infrared laser as described above, which has a light emitting region in the infrared range, as an exposure light source, an image is formed by using, for example, a photopolymerizable composition in its recording layer.
As the existing methods of forming an image by light exposure with the use of a photopolymerizable composition, there have been known various methods, for example, a method which comprises forming a recording layer with the use of a photopolymerizable composition containing an ethylenically unsaturated compound and a photopolymerization initiator on the surface of a support, carrying out image-exposure to thereby polymerize and harden the ethylenically unsaturated compound in the exposed part and then solubilizing and removing the unexposed part to thereby form a hardened relief image, a method which comprises causing a change in the pressure-sensitive adhesion strength of a photopolymerizable composition layer (a recording layer) by light exposure and then peeling off the support to thereby form an image, a method of forming an image by taking advantage in a change in the toner-adhesion properties of a photopolymerizable composition layer caused by light, and soon. As the photopolymerization initiators in these methods, it hap been a practice to employ compounds sensible to short-wavelength rays mainly in the ultraviolet range of 400 nm or less such as benzoin, benzoin alkyl ethers, benzyl ketal, benzophenone, anthraquinone, benzyl ketone and Michler's ketone.
With the recent development in the image-forming techniques, on the other hand, sensitive materials showing high sensitivity to lights in the visible range have been strongly required. For example, a large number of photopolymerizable compositions having widened sensitivity ranges, i.e., around 500 nm are proposed for the laser plate making system using argon ion laser oscillation beam of 488 nm. Moreover, studies are vigorously made on photopolymerizable compositions to lights in the longer wavelength range exceeding 600 nm appropriate for the laser plate making system with the use of He—Ne lasers or semiconductor lasers and full-color image replication techniques.
There have been known photopolymerizable compositions containing an ethylenically unsaturated compound and a photopolymerization initiation system in which heterocycles are linked via a mono-, tri-, penta- or heptamethine chain in the photopolymerization initiation system and which contain a cyanine colorant having a specific structure and an s-triazine derivative having a specific structure (see, for example, JP-A-58-29803 and JP-A-4-31863). Also, there have been proposed photopolymerizable compositions in which a photopolymerization initiator system comprises a squarylium compound having a specific structure and a specific s-triazin compound (see, for Trample, JP-A-4-106548).
However, it is known that a photopolymerization initiator usually shows a rapid decrease in the sensitivity in the ability to emit active radicals to rays having wavelengths of 500 nm or more, in particular, rays having wavelengths of 600 nm or more. None of the above-described photopolymerizable compositions having been proposed has a satisfactory sensitivity to these rays in the longer wavelength range. In addition, there is another problem that the photopolymerization reaction would proceeds when handled under a white fluorescent lamp. Namely, a photopolymerizable composition having stable qualities can be hardly obtained at present.
In order to elevate the sensitivity of the above-described photopolymerizable compositions and improve the handling properties thereof under a white fluorescent light, photopolymerizable compositions containing an ethylenically unsaturated compound, a specific pigment and a photopolymerization initiator (a triazine compound or the like) have been proposed (see, for example, JP-A-2000-131837).
However, a lithographic printing plate having such a composition in the recording layer shows a low stability in preserving the printing plate. Therefore, it has been urgently required to elevate the sensitivity while maintaining a practically satisfactory stability.